1. Field of the Invention
The present invention relates to a cathode-ray tube such as a color picture tube and, more particularly, to a cathode-ray tube having a flat face plate, a flat shadow mask opposing a phosphor screen formed on the inner surface of the face plate, a mask support member supporting the shadow mask, and a plate support member supporting the face plate and a rear plate, wherein the phosphor screen has a plurality of regions which are scanned independently of one another.
2. Description of the Related Art
Recently, various studies have been conducted on high-definition broadcasting and a high-resolution picture tube with a large screen designed for the high-definition broadcasting. In general, in order to achieve high resolution of a picture tube, the spot diameter of an electron beam on a phosphor screen must be reduced.
For this purpose, in the prior art, the structure of an electron gun electrode has been improved, or the caliber and/or length of the electron gun has been increased. However, satisfactory achievement has not been obtained. The main reason is that the distance between the electron gun and the phosphor screen increases in accordance with the increase in size of the picture tube, and the magnification of the electron lens increases excessively. Accordingly, in order to achieve high resolution, it is important to shorten the distance (depth) between the electron gun and the phosphor screen. In addition, when the deflection angle of an electron gun is increased, the difference in magnification between the center area and peripheral area of the phosphor screen increases. Thus, wide-angle deflection is not advantageous for achieving high resolution.
Under the circumstances, in the prior art, as disclosed as in Jpn. Pat. Appln. KOKAI Publication No. 48-90428, there is known a method of arranging a plurality of independent small-sized picture tubes, thereby constituting a high-resolution, large screen. This kind of method is effective for large-scale screen display with a large number of divided regions, which is designed for outdoor installation. However, when this method is applied to middle-scale screen display (e.g., the screen size is about 40 inches), connection portions between the divided regions of the screen are conspicuous, resulting in low-quality images. Thus, when the display formed by this method is used as a household TV receiver or computer-aided design (CAD), the connection portions on the screen are a serious defect.
On the other hand, U.S. Pat. No. 3,071,706 discloses a picture tube wherein a plurality of independent picture tubes are continuously arranged and the screens of these picture tubes are integrated. According to this picture tube having the integrated phosphor screen, a vacuum envelope is constituted by a face plate having an inner surface coated with a phosphor screen, a rear plate opposed to the face plate, funnels adjacent to the rear plate, and necks provided on the funnels.
In the case of this structure of the envelope, however, if the screen surface becomes broader, it is necessary to increase the thickness of the face plate or rear plate in order to withstand the load of atmospheric pressure (external pressure). In addition, it is necessary to provide the face plate with a high curvature in the tube axis direction. As a result, the weight of the envelope becomes considerably heavy, and moreover the screen of the picture tube with the face plate having such a high curvature in the tube axis direction cannot be viewed clearly. In addition, the distance between the phosphor screen and the electron gun sealed within the neck increases, and the magnification of the electron lens is adversely affected.
In order to solve the problems posed in the picture tube having the above integrated phosphor screen, Jpn. Pat. Appln. KOKAI Publication No. 5-36363 discloses a picture tube in which both a face plate and a rear plate are formed to be flat, and an integrated phosphor screen formed on the inner surface of the face plate is dividedly scanned by electron beams emitted from a plurality of electron guns. In this picture tube, plate support members are arranged inside a vacuum envelope to support the load of atmospheric pressure applied to the flat face plate and the flat rear plate.
When, however, the above structure is applied to a color picture tube having a shadow mask, the shadow mask, which is arranged to oppose the phosphor screen, must also be formed to be flat. For this reason, the following problems are posed.
First, there is a problem in the method of attaching the shadow mask. Specifically, in the case of a conventional color picture tube having a spherical face plate, the shadow mask is also spherical. In this case, by fixing a peripheral portion of the shadow mask to a metallic frame (mask frame), practical mechanical strength can be given to the shadow mask and it becomes easy to situate the shadow mask in a predetermined positional relationship with the phosphor screen formed on the inner surface of the face plate. However, in the case of a flat face plate, the shadow mask must also be flattened, and therefore the mechanical strength of the shadow mask is low. Accordingly, this shadow mask cannot easily be situated in a predetermined positional relationship with the phosphor screen only by fixing a frame to the peripheral portion of the shadow mask to reinforce the mask, as in the prior art.
In general, regarding a flat shadow mask or a cylindrical shadow mask which has a curvature only in one direction, sufficient mechanical strength is given to the shadow mask by fixing it to a robust frame with a tensile force applied to the shadow mask, and the shadow mask can be situated in a predetermined positional relationship with the face plate via the frame. A color picture tube having such a structure is disclosed in, e.g., Jpn. Pat. Appln. KOKAI Publication No. 2-158544, in which one large funnel is connected to the face plate.
In this structure, however, with an increase in screen size, the tensile force applied to the shadow mask must be increased accordingly. Consequently, a more robust frame is required. In this case, not only the weight of the entire picture tube increases, but also the attaching means for attaching the shadow mask to the face plate via the frame must have a complicated structure. Furthermore, a sufficient space for providing the attaching means is required.
Second, there is a problem in mounting precision of the shadow mask. A phosphor screen of a regular color picture tube is formed by exposing a phosphor screen material layer such as a phosphor slurry coated on the inner surface of a face plate by a photo-engraving method using a shadow mask incorporated in the color picture tube as a photomask. If, therefore, the distance (q-value) between the shadow mask and the inner surface of the face plate is deviated from a predetermined value, the arrangement pitch of phosphor layers is affected but the continuity of the entire phosphor screen is not affected.
On the other hand, in the case of a color picture tube wherein an integrated phosphor screen has a plurality of regions which are scanned independently of one another, a plurality of effective portions in which a large number of electron beam passage apertures are formed are discontinuously arranged via ineffective portions having no electron beam passage apertures in correspondence with a plurality of regions of the phosphor screen. For this reason, in a color picture tube of this type, the influence of the q-value appears between adjacent regions of the phosphor screen. More specifically, when the q-value is greater than a predetermined value, phosphor layers on adjacent regions of the phosphor screen overlap one another; when the q-value is less than a predetermined value, a gap is produced between phosphor layers on adjacent regions.
In addition, when a phosphor screen is formed by a so-called master mask method using a photomask or a dry plate, the q-value must be accurately set. According to the master mask method, a phosphor screen having continuity can be accurately formed. If, however, the q-value is not exact, an electron beam does not land on a predetermined phosphor layer, i.e., so-called miss-landing occurs, when a color picture tube is assembled. In addition, rasters between adjacent regions overlap one another, or a gap is produced between the rasters.
Furthermore, disregarding the formation of the phosphor screen, the required precision of the q-value is about 0.05 mm, though it depends on the horizontal deflection angle or the arrangement pitch of electron beam passage apertures of the shadow mask. As can be seen from the fact that the required manufacturing precision of the conventional color picture tube is about 0.5 mm, very high precision is required of the q-value. For this reason, in a color picture tube in which one integrated phosphor screen formed on the inner surface of a flat face plate has a plurality of regions which are scanned independently of one another, it is substantially impossible to mount a shadow mask by the conventionally known means.
Third, there is a problem in deformation and vibration in a shadow mask. A flat shadow mask is susceptible to deformation and vibration. When the shadow mask is deformed, the q-value varies, thus causing miss-landing. In addition, when the shadow mask is vibrated, miss-landing also occurs because the q-value changes with time.
As a plate support member arranged in a vacuum envelope to support the load of atmospheric pressure applied to a flat face plate and a flat rear plate, a plate support member having a needle- or wedge-shaped distal end portion in contact with the face plate or a plate support member having a plate-like shape as a whole is available. It is desirable that each of these plate support members be situated outside the locus of electron beams scanning the phosphor screen, and be reduced in size as much as possible. When the distal end portion of each plate support member is formed to be narrow, and the number of plate support members arranged is reduced, the load of atmospheric pressure applied to each support member increases. In addition, when a plurality of plate support members have different heights, the deformation of the face plate is increased by the load of atmospheric pressure, resulting in a deterioration in reliability with respect the resistance to atmospheric pressure.
As described above, in a color picture tube in which one integrated phosphor screen formed on the inner surface of a flat face plate has a plurality of regions which are scanned independently of one another, since a shadow mask arranged to oppose the phosphor screen must also be formed to be flat, problems are posed in terms of a method of attaching the shadow mask, mounting precision of the shadow mask, deformation of the shadow mask, and the like. Especially in a large-sized color picture tube, it is very difficult to arrange a shadow mask with high precision. In addition, it is difficult to realize a simple, lightweight means for mounting the shadow mask. Furthermore, a flat shadow mask is extremely susceptible to deformation and vibration.